1. Field of the Invention
The present invention relates to copper base alloys, and to methods for producing a casting and a forging employing these copper base alloys.
This application is based on Japanese Patent Application No. 2000-103662, the contents of which are incorporated herein by reference.
2. Description of the Related Art
Metallic materials that are high strength and high thermal conductivity are employed in fields where materials are subjected to severe thermal fatigue, such as in the case of structural materials of forming nuclear fusion reactors and thrust chambers of a rocket engine where one surface is in contact with 3000° C. combustion gas and the other surface is in contact with liquid hydrogen.
A copper base alloy containing 0.8% Cr and 0.2% Zr (note that “%” as employed in this specification hereinafter indicates mass %) that is disclosed in Japanese unexamined Patent Application, First Publication No. Hei 04-198460 may be cited as an example of a high strength high thermal conductivity alloy that is employed in these fields. In general, a high strength, high thermal conductivity forging can be obtained from this copper base alloy by casting it, and then forming it into a specific shape by forging, rolling, etc. while applying a specific heat treatment thereto. In this copper base alloy, it is possible to increase the tensile strength while maintaining thermal conductivity at a high level, even if the alloy's composition is the same, by adjusting the conditions of the thermomechanical treatment.
In recent years, however the conditions under which structural components are employed have become severe with respect to the generation of thermal stress. At the same time, the short lifespan of conventional materials before cracking occurs has been pointed out. Thus, there has been a demand for higher resistance to thermal fatigue. To reduce thermal strain in metallic materials, it is necessary to increase thermal conductivity as well as to improve thermal fatigue strength. Improvements in thermal conductivity are near the limits, however. Thus, the challenge is to improve thermal fatigue strength without reducing thermal conductivity as compared to conventional metal materials.
It is known that in order to increase thermal fatigue strength in these types of metal materials, it is generally acceptable to increase tensile strength and tensile proof stress without reducing tensile elongation and thermal conductivity at the employed temperatures. Therefore, in order to meet the aforementioned demands, an attempt was made to increase strength by employing a copper base alloy that contained Cr (0.8%) and Zr (0.2%) as the base, and then increasing the draught of the copper base alloy by further increasing the Cr and Zr contents.
In this type of Cu—Cr—Zr alloy, a high degree of strength can be obtained if the Cr and Zr contents are increased, while at the same time generating a fiber-type microstructure by swaging or wire drawing, which apply a large deformation in one direction.
Ductility is reduced in this type of Cu—Cr—Zr alloy, however, so that thermal fatigue strength did not improve as much as expected. Moreover, there are limitations on the shape of the formed article, so that a sufficient amount of forging and rolling cannot be carried out. Thus, it was difficult to obtain the desired strength for a formed article of an optional shape. Accordingly, applications were limited to electrical parts utilizing high strength and electroconductivity.
On the other hand, a Cu—Ag alloy in which a large amount of Ag is added has been developed as a new alloy, as disclosed in Japanese Unexamined Patent Application, First Publication No. Hei 6-279894 and in SAKAI, et al: J. JAPAN INST. METALS, Vol. 55, No. 12 (1991), pp. 1382–1391. Ag, like Cr and Zr, has little solubility in Cu near room temperatures, and experiences little reduction in thermal conductivity when rendered into an alloy. However, if Ag is added in an amount of 8.5% or more, then the obtained copper base alloy forms eutectic when solidifying. Thus, if swaging or wire drawing, which apply a large deformation in one direction, are carried out in the same manner as in the case of a Cu—Cr—Zr alloy to an ingot of a Cu—Ag alloy in which 15% Ag has been added in order to obtain a sufficient amount of eutectic structure, the eutectic structure is destroyed and a fiber reinforced structure is generated. The strength obtained in this case is extremely high.
However, in the case of this type of Cu—Ag alloys, sever working such as to obtain a wire rod with a 1/10 or smaller diameter from a forged round bar is required. Thus, it is not possible to produce wrought articles of greater than a certain degree of thickness with this technology.
In addition, in the above-described metal materials, repetition of forging and heating treatments increase production costs. Accordingly, since strength on par with current levels is sufficient, there has been a desire for a metal material that is high thermal conductivity, high strength, and inexpensive that can be produced by means of casting where a forging step is not required. However, this type of metal material has not been conventionally known.